In a wiring circuit substrate used in an electronic/electric device or the like, a terminal for connection to an external circuit is formed. As such a terminal, there is known a flying lead (hereinafter, abbreviated as FL) in which not only one side of a conductive layer but also both sides thereof are used as a terminal in order to cope with high density and miniaturization of a recent electronic/electric device.
For example, in a suspension substrate with circuit, which is a wiring circuit substrate used in an HDD, an FL may be included as a terminal. A suspension substrate with circuit, which has an FL, includes a metal support layer made of stainless steel, a first insulating layer formed on the metal support layer and made of polyimide, a conductive layer for wiring formed on the first insulating layer, used as a wiring and made of copper, and a second insulating layer formed on the conductive layer for wiring and made of polyimide.
In such a suspension substrate with circuit, an FL is formed by: exposing a front surface of the conductive layer for wiring by opening the second insulating layer and simultaneously exposing a rear surface of the conductive layer for wiring by opening the metal support layer and the first insulating layer; and performing Ni plating or Au plating and the like on both exposed surfaces of the conductive layer for wiring. The FL formed in the above manner is connected to a terminal of an external circuit by applying ultrasonic vibration by using, for example, a bonding tool.
However, since such an FL is provided with the exposed conductive layer for wiring made of copper, there have been problems in that mechanical strength is weak, and a wiring is easily disconnected due to stress concentration on the exposed conductive layer for wiring during ultrasonic bonding.
In Patent Literature 1, as a method for coping with such problems, there is proposed a technique that reinforces mechanical strength of an FL in such a manner that an edge section in an opening exposing a conductive layer for wiring constituting the FL, and a wide section expanding in a width direction substantially perpendicular to an extending direction of the conductive layer for wiring in a portion intersecting with the conductive layer for wiring are formed on the conductive layer for wiring.
Moreover, in Patent Literature 1, there is also proposed a technique that reinforces mechanical strength of an FL in such a manner that, in a first insulating layer and a second insulating layer, a protrusion protruding from the edge section side of the above-described opening to a central section side is formed along front and rear surfaces of the conductive layer for wiring exposed to the opening.
Also, in Patent Literature 2, there is proposed a technique that reinforces mechanical strength of an FL in such a manner that, in a conductive layer for wiring constituting the FL, a thickness of a portion exposed to the above-described opening is made thicker than a thickness of a portion not exposed to the opening by a copper plating method or the like.
As described above, in the suspension substrate with wiring circuit having the FL proposed in Patent Literatures 1 and 2, the mechanical strength of the FL is reinforced. Therefore, in these suspension substrates with wiring circuit, the wiring disconnection of the FL is prevented when bonding the FL and the terminal of the external circuit, leading to an improvement in connection reliability therebetween.
On the other hand, in a flexible substrate for suspension used in an HDD, a terminal section for connection to an external circuit is formed for connection to the external circuit. As such a terminal section for connection to the external circuit, there is known an FL in which a wiring conductive layer, whose front surface and rear surface are exposed, is used in order to cope with high density and miniaturization of a recent electronic/electric device.
The mechanical strength of the FL may be insufficient because the front surface and the rear surface of the FL are exposed, and the wiring conductive layer used in the FL is made of copper (hereinafter, abbreviated as Cu). For this reason, when the FL is connected to the terminal of the external circuit side by the bonding tool such as ultrasonic bonding, problems such as the wiring disconnection of the FL have occurred. In order to solve such problems, there are proposed the flexible substrates for suspension having the FL proposed in Patent Literatures 1 and 2.